This invention relates to a radiation-curable adhesive formulation useful for bonding together surfaces of digital versatile discs.
The compact disc, or CD as it is commonly known, revolutionized the recording and computer industries, making the storage of enormous amounts of data, such as music, possible in an inexpensive, readily available medium. The technology behind the compact disc has been improved and expanded to meet the increasing storage needs of the computer and entertainment industries, culminating in the creation of digital versatile discs, or DVDs. While compact discs and digital versatile discs store information in the same general manner, the DVD design exploits CD technology to create a superior product.
Structurally, digital versatile and compact discs are very similar to one another. The information bearing surfaces of both discs are marked with indentations, or pits, arranged in a continuous spiral pattern. As the drive laser moves across the pits, the laser beam is reflected back to the driver, which receives the light signal and converts it into an appropriate format, for example, audio, video, graphic or textual format. DVDs store more data than equivalent CDs because, inter alia, the information-carrying pits are smaller and are intimately spaced in tight tracks, as opposed to the wide tracks of CDs. DVD players utilize lasers which emit red light at 650 nm and 635 nm, which are shorter wavelengths than the infrared light used in conventional CD players. These shorter wavelengths enable DVD players to accurately read the smaller, more densely packed pits of the digital versatile discs.
The compact and digital versatile discs, composed of a core member around which the information bearing surface is symmetrically arranged, are the same diameter (120 mm), and the same thickness (1.2 mm). However, instead of a single layer characteristic of traditional compact discs, digital versatile discs are made of two 0.6 mm layers of polycarbonate. This reduces the amount of distance between the surface of the discs and the pits, such that the laser penetrates less plastic in the DVDs than in CDs when accessing information. Consequently, the thinner DVD substrate enhances the read accuracy of the laser. The two bonded sides of the DVDs serve to strengthen the discs, preventing warping. Thus, digital versatile discs have greater capacity and reduced responses to environmental factors than compact discs.
Digital versatile discs may be created by variations on a few basic processes, as disclosed, for example, by U.S. Pat. Nos. 4,310,919 and 4,423,137. For example, during production of digital versatile discs, a master glass disc with the desired information is created, using a laser beam to record data from the center of the master glass disc to the outer edge of the master glass disc in a spiral pattern. After recording, the master glass disc is developed by spinning a sodium hydroxide solution over the glass surface, revealing the pits created by the laser. The developed master glass disc is then metallized with a coating of silver, followed by a coating of nickel. The nickel layer is then separated from the silver-coated master glass disc, forming a nickel reverse image of the data, known as the father copy. One or more nickel copies of this father may be generated, which can be used as a stamper in an injection molding machine to mass produce discs. Molten polycarbonate is then shot into molds containing the stamper, creating polycarbonate discs carrying the desired information. The discs are then removed from the molds with the lacquer layer adhered thereto, and a reflective metal, usually aluminum, is evaporated or sputtered on top of the polycarbonate first layer containing the information. A protective coating of lacquer is then applied over the reflective layer and dried or cured, forming a single sided disc. The stamped side of the single-sided DVD is backed by a dummy layer, onto which graphics may be applied.
The basic DVD configuration is usually modified to further enhance the capacity of the discs. The capacity of a single sided disc may be almost doubled by applying a semi-reflective data layer zero, comprising, for example, gold, over the reflective aluminum layer one. The gold layer may be read by the driver laser on a low power setting, while the aluminum layer may be accessed by increasing the power of the laser. This results in a double layer of information on a single side of a disc, imparting the DVD with currently about 8.5 GB of capacity.
Two of these single sided, dual layer discs may be bonded together back to back with a thick layer of adhesive, creating double sided, double layer digital versatile discs with currently about 17 GB of storage space. The first and second disc layers are bonded such that they are parallel to and equidistant from the core member of the disc. The adhesive employed must provide high shear strength, while keeping the information layers uniformly equidistant from each other.
Three technologies are currently employed for DVD bonding, namely contact adhesives, cationic or PSA UV bonding, and free radical UV bonding. The formulations must provide adhesion between the aluminum and polycarbonate layers, the gold and polycarbonate layers, and the lacquer and the polycarbonate layers, and various combinations thereof. Furthermore, the adhesive coatings must have a high cure speed and must wet the substrate. Following cure, these materials must have high dimensional stability and durability.
However, strong, long-lasting adhesion between DVD component layers, without compromising the other desirable properties, such as dimensional stability of the disc, is not achievable with the existing systems.
Contact adhesives are applied to discs in a hot melt process, during which temperatures are kept between 120xc2x0 C. and 160xc2x0 C. The adhesive is spread on the discs as a thin layer by roll coating both inner bonding surfaces. The halves are then pressed together and the adhesive is allowed to set. Flat discs may be produced at high yield rates via this method, but these discs tend to warp when stored above 70xc2x0 C. or in humid environments.
During cationic UV bonding, the adhesive is screened onto both the discs, UV irradiated, and then pressed together. The bond strengthens with time due to aging, such that after approximately 24 hrs, the disc halves are permanently attached to each other. The discs produced by this method are flatter than with other processes, but cationic UV bonding necessitates an additional lacquer coating step. Additionally, the discs must stay in a curing station for a period to ensure complete bonding prior to stacking, requiring an extra stacker, which increases equipment costs.
During free radical UV bonding, acrylate lacquer is placed on the leading edge of a disc, after which a second disc is placed on top, and the pair is spun. The weight of the second disc promotes the movement of the lacquer toward the inner edge of the metal layer, while the spinning causes the lacquer to move to the outer edge. The adhesive is cured via UV irradiation after the spin coating process is completed. Radical UV bonding is prone to bubble formation between the bonded layers. In the dual layer construction, bubbles can impair the ability of the drive laser to read the information-bearing pits. Variations in the aluminum layer prior to bonding can cause uneven curing, which prevents the formation of flat discs. Furthermore, acrylates shrink upon cure, often to substantial degrees, thereby preventing the formation of flat discs. This shrinkage may also reduce the environmental stability of bonded discs.
The object of the current invention is a an adhesive, that binds sputter-coated metallized or siliconized, polycarbonate substrates to UV-cured lacquer surfaces, that is stable following exposure to elevated temperature and humidity, possesses excellent mechanical properties, has suitable viscosity, acceptable shrinkage and has a low degree of volatility post-cure. The result is an adhesive that imparts impact resistance and superior shear strength to bonded digital versatile discs or to other substrates.
The object of the invention is achieved by a UV or radiation-curable composition for use as an adhesive material comprising the combination of the following pre-mixture ingredients:
(A) about 5 wt. % to about 80 wt. % of at least one UV or radiation-curable acrylate oligomer;
(B) about 10 wt. % to about 20 wt. % of at least one non-acrylate functional reactive diluent;
(C) about 10 wt. % to about 80 wt. % of at least one acrylate functional reactive diluent;
(D) about 0.5 wt. % to about 10 wt. % of at least one radical forming sulphur compound, and
(E) optionally about 0.1 wt. % to about 15 wt. % of one or more photoinitiators,
wherein the xe2x80x9cpre-mixture ingredientsxe2x80x9d correspond to the identity of radiation-curable composition components prior to mixture with other ingredients.
The present invention provides for the production of an improved adhesive for bonding digital versatile discs, methods for bonding disc components together, and discs with improved impact resistance due to the enhanced bonding properties of the adhesive compound.
The radical forming sulphur compound generally will be a thiol compound or a polysulphide compound. Hereinafter, mostly it is referred to thiol compound, but this is just as an example.
Acrylate oligomers are well known in field of adhesives. According to the invention, it is supposed, that co-polymerization of thiol and non-acrylate functional compounds (sometimes mentioned as xe2x80x9cenexe2x80x9d) with urethane acrylates creates a urethane-acrylate-thiol-ene hybrid adhesive coating with superior properties to urethane acrylate coatings lacking the thiol-ene system. The non-acrylate functional compound can e.g. be an acrylamide or an N-vinyl group comprising compound. Because standard adhesive materials do not provide strong, long lasting bonding between, for example, aluminum and polycarbonate substrates, especially under adverse environmental conditions, particularly elevated temperatures and humidity levels, the creation of hybrid acrylate-thiol-ene adhesive formulations is a marked improvement over the current methodology.
Although not certain, the thiol-ene systems appear to allow copolymerization of non-acrylate functional moieties with acrylate moieties. In the absence of thiols, copolymerization of, for example, N-vinyl compounds is slow. Thiol compounds act as chain transfer agents, which may reduce cure speed. In contrast, thiol-ene systems in acrylate compounds enhance cure speed and reduce shrinkage of cured adhesives during cure of adhesive films.